Alkyl Side Chain Engineering for Difluorinated Benzothiadiazole Flanked Non-Fullerene Acceptors Toward Efficient Polymer Solar Cells

Side chain engineering plays a vital role in fine tuning of non-fullerene acceptors for the contemporary organic solar cells. In this work, we compare central alkyl side chains in the modification of photophysical and photovoltaic properties of difluorinated benzothiadiazole flanked non-fullerene acceptors. With a hexyl side chain, the resulting non-fullerene H-FFBR possesses more compact intermolecular interaction, leading to narrower bandgap, higher-lying HOMO level and stronger self-aggregation, if compared to O-FFBR with octyl side chain. For the photovoltaic performances, with PTB7-Th as the donor polymer, the O-FFBR-based solar cells exhibit power conversion efficiency of 9.4%, which is higher than that of 9.1% for the H-FFBR-based devices. Bulk-heterojunction morphologies of the O-FFBR- and H-FFBR-based active layers were compared by atomic force microscopy (AFM) and transmission electron microscope (TEM), revealing that O-FFBR is likely to give lower surface roughness and smaller phase separation. Our work demonstrates that varying alkyl side chain of a non-fullerene acceptor is valuable to tune the bulk-heterojunction morphology and resulting photovoltaic performance.